1. Field of the Invention
The present invention relates to new crystalline zeolite SSZ-41 prepared using a diquaternary ammonium compound comprising a linear polymethylene group having heterocyclic rings bonded to each end of the polymethylene group, each heterocyclic ring containing one or more nitrogen atoms with one nitrogen atom in each ring being quaternary ammonium (hereinafter referred to as the "polymethylene diquat compound, a new method for preparing SSZ-41, and hydrocarbon conversion processes using SSZ-41 as a catalyst.
2. State of the Art
In conventional usage the term "molecular sieve" refers to a material having a fixed, open-network structure, usually crystalline, that may be used to separate hydrocarbons or other mixtures by selective occlusion of one or more of the constituents, or may be used as a catalyst in a catalytic conversion process. The term "zeolite" refers to a molecular sieve containing a silicate lattice, usually in association with some aluminum, boron, gallium, iron, and/or titanium. In the following discussion and throughout this disclosure, the terms molecular sieve and zeolite will be used more or less interchangeably. One skilled in the art will recognize that the teachings relating to zeolites are also applicable to the more general class of materials called molecular sieves.
Natural and synthetic crystalline molecular sieves are useful as catalysts and adsorbents. Each crystalline molecular sieve is distinguished by a crystal structure with an ordered pore structure, and is characterized by a unique X-ray diffraction pattern. Thus, the crystal structure defines cavities and pores which are characteristic of the different species. The adsorptive and catalytic properties of each crystalline molecular sieve are determined in part by the dimensions of its pores and cavities. Accordingly, the utility of a particular molecular sieve in a particular application depends at least partly on its crystal structure.
Because of their unique sieving characteristics, as well as their catalytic properties, crystalline molecular sieves are especially useful in applications such as hydrocarbon conversion, gas drying and separation. Although many different crystalline molecular sieves have been disclosed, there is a continuing need for new zeolites with desirable properties for gas separation and drying, hydrocarbon and chemical conversions, and other applications.
Crystalline aluminosilicates are usually prepared from aqueous reaction mixtures containing alkali or alkaline earth metal oxides, silica, and alumina. Crystalline borosilicates are usually prepared under similar reaction conditions except that boron is used in place of aluminum. By varying the synthesis conditions and the composition of the reaction mixture, different zeolites can often be formed.
Organic templating agents are believed to play an important role in the process of molecular sieve crystallization. Organic amines and quaternary ammonium cations were first used in the synthesis of zeolites in the early 1960s as reported by R. M. Barrer and P. J. Denny in J. Chem. Soc. 1961 at pages 971-982. This approach led to a significant increase in the number of new zeolitic structures discovered as well as an expansion in the boundaries of composition of the resultant crystalline products.
Previously, products with low silica to alumina ratios (SiO.sub.2 /Al.sub.2 O.sub.3 .ltoreq.10) had been obtained, but upon using the organocations as components in the starting gels, zeolites with increasingly high SiO.sub.2 /Al.sub.2 O.sub.3 were realized. Some of these materials are summarized by R. M. Barrer 1982, Hydrothermal Chemistry of Zeolites, New York: Academic Press, Inc.
Unfortunately, the relationship between structure of the organocation and the resultant zeolite is far from predictable, as evidenced by the multitude of products which can be obtained using a single quaternary ammonium salt as reported by S. I. Zones et al., 1989, Zeolites: Facts, Figures, Future, ed. P. A. Jacobs and R. A. van Santen, pp. 299-309, Amsterdam: Elsevier Science Publishers, or the multitude of organocations which can produce a single zeolitic product as reported by R. M. Barrer, 1989, Zeolite Synthesis, ACS Symposium 398, ed. M. L. Occelli and H. E. Robson, pp. 11-27, American Chemical Society.
Thus, it is known that organocations exert influence on the zeolite crystallization process in many unpredictable ways. Aside from acting in a templating role, the organic cation's presence also greatly affects the characteristics of the reaction mixture gel. These effects can range from modifying the gel pH to altering the interactions of the various components via changes in hydration (and thus solubilities of reagents) and other physical properties of the gel. Accordingly, investigators have now begun to consider how the presence of a particular quaternary ammonium salt influences many of these gel characteristics in order to determine more rigorously how such salts exert their templating effects.
In summary, a variety of templates have been used to synthesize a variety of molecular sieves, including zeolites of the silicate, aluminosilicate, and borosilicate families. However, the specific zeolite which may be obtained by using a given template is at present unpredictable. In fact, the likelihood of any given organocation serving as an effective template useful in the preparation of a molecular sieve is conjectural at best. In particular, organocation templating agents have been used to prepare many different combinations of oxides with molecular sieve properties, with silicates, aluminosilicates, aluminophosphates, borosilicates and silicoaluminophosphates being well known examples.
M. J. Annen and M. E. Davis, Microporous Material, Vol. 1, No. 1, February 1993 (pp. 57-65) discloses a zeolite designated VPI-8. From the X-ray pattern provided by Annen and Davis, it appears that VPI-8 has a crystal structure similar to SSZ-41. VPI-8 is described as a high-silica molecular sieve; elemental analysis of ammonium-exchanged VPI-8 giving the following molar composition: 0.009 Zn:1.00 Si:0.026 Li. It is further stated that VPI-8 reversibly adsorbs 0.04 g/g argon at 87.degree. K. VPI-8 was prepared from a gel having the following composition: 0.44 Li.sub.2 O:0.3 ZnO:SiO.sub.2 :44 H.sub.2 O.
A new zeolitic material, designated SSZ-41, has now been discovered. It has a structure similar to VPI-8, but differs from VPI-8 in that SSZ-41 has an argon adsorption capacity greater than (e.g., up to about three times) that reported for VPI-8, and SSZ-41 may contain aluminum (whereas VPI-8 has none).